Acoustic apparatus



Feb. 12, 1929.

L. DE FOREST ET AL ACOUSTIC APPARATUS Filed June 30, 1926 VVENTOR Leaafar 7 BY if? L- Fz nMw s m @L 130 Wm: ATTORMEZS.

Patented Feb. 12, 1929.

UNITED- 1,701,911 PATENT OFFICE.

LEE DE FOREST, OF NEW YORK, AND JOHN LOUIS REYNOLDS, F ASTORIA, NEWYORK,

ASSIGNOBS, BY MESNE ASSIGNMENTS, T0 GENERAL TALKING- PICT'U'BES CORPORA-TION, A CORPORATION OF DELAWARE.

' ACOUSTIC APIABATUS.

Q e pplication flled .l'une 30, 1926. Serial'No. 119,777.

This invention relates to improvements soundreproducing apparatus usedin con- 'Iiection with auditoriums and the like.

One of the objects of thisinvention is the improvement in the circuitsused in connection with sound reproducing devices whereby any soundreproduction may be varied or adapted at will to the acousticrequirements oi an auditorium.

Another object of this invention is theprovision of a combination ofapparatus for providing voice and musical productions in au ditoriums inconnectionwith the projection of amotion picture film.

j A still further object of the invention is the combination of a motionpicture projection machine and 'a\phonofilm attachment with means forvarying the sound reproductions in accordance with the character of thesound being produced in order to adapt it for use with the auditorium.

These and many other obpects are successfully sought by this inventionas will appear hereinafter.

This invention resides substantially in the construction, arrangement,combination and relative location of parts and circuit assemblies, aswill'be clearly described below.

, Referring to the drawings in which the preferred embodiments of myinvention are disclosed; (Figure 1 represents in diagrammatic form thegeneral arrangement of the elements of our invention.

Figure 2 is a diagrammatic showing of the circuits used in connection.with a modification of our invention.

Our invention relates particularly to the combination of an amplifierand sound reproducing device for theatre work, where the amplifier isadvantageously located the projection booth of the motionpicture theatreand the'loud speaker is located near the projection screen in theauditorium, and at some distance from the motion picture proof differentsizes and arrangements, the acous;

tic requirements to give the best'reproduc-i of greater volume than thehigher pitched tones produce in frequent instanees, overpowering echoeswhich render it dlfiicult for jection booth. We find that in auditoriumspeople located at distances from the horns 65 to clearly understand. Insuch cases, it is important to reduce the volume of the reproduction incertain passages of the sound rec- 0rd to a greater degree than at othertimes. For example, when an orchestral number (musical) is beingreproduced the deep tones add richness and beauty to the record and itis possible under such circumstances that the low pitched tones may bereproduced to ,7, advantage in full volume; whereas, in a speakingnumber where it is important that every word he clearly understood, wefind that it is'advantageous to reduce the sound volume duringsuchkpassages and particularly the volume of the low pitched sounds. mInasmuch as it is impossible for the operator in the projection booth toclearly hear what is going on from the horns at the other end of theauditorium due to the loud noises inseparable from the operation of theprojection machine, it is highly desirable that the controlling meansfor regulating at will the sound volume be located in the auditorium ata dista'nce from the projection booth. Such an arrangement is shown inFigure 1, where 8 1 represents the walls of the projection boothcontaining the projection machine, 3, equipped with the phonofilmreproducing attachment 4, such as described in the co-' pendingapplication of Lee de Forest, Serial 5 No. 635,334, filed April 28th,1923, connected to the amplifier 5 and projecting the picture 7 throughthewindow 2 in the wall of the projection bogth, upon the screen 10located at the other end of the auditorium. The loud 9 speaker horn 9isshown in this case located underneath the screen 10 and is connected tothe output of the amplifier 5 by means of the walls 6 in one of which isincluded the variable resistance 8.

By means of thearrangement here shown, some competent person located theauditorium, as for example, the orchestra con-. ductor hasit within hispower to readily control at will the volume of the sound reproduction tosuit the acoustic requirements of that particular auditorium and tomodulate the particular number which is being reproduced to give thebest and most intelligible results. '105 By the arrangement shown inFigure 1, however, it is only possible to reduce all, soundfrequenciesalike without discrimina- 4o tance 29' and e resistance isknown as an 7.

" tion. It is. highly desirable in sadism to this toamplify at will thehigher frequencies :record'is projected. This cell is energized by of asoun record or to diminish the volume of the lower frequencies withoutat the same tlme that of the higher frequency sound. m a An arr ment fordoing this is shown in Figure. 2, in the two stage amplifier circuit.

In this figure, 20 is the photo-electric cell into which thelight fromthe photographic sound 1 means of the battery 21 and the two in seriesare coupled to the input circuit of the vacuum tube 26 by means of theinput resistance .22.

.ries with the ,tube'27 is connects A condenser 23 is inserted 1n thelead to the grid and a high resistance grid 'lead2 in s'eed'across thegrid and film of the vacuum tube 26. The output-of this tube isconnected through the primary 31- of a transformer and a battery 34.Connected across the terminals of the primary 31 is an inductance 28having connected in one lead thereof the variable resistance 30. Thevariable capacitance 29 is shunted across the'inductance 28.

The secondary 32 of the transformer is bridged by means of. thepotentiometer 33.

A lead from "one terminal of. the secondary is connectedthroughthecondenser 35 to one of the film leads. The

tiometer 33 b means'of the adjustable connection40. T e output or platecircuit of the the sound reproducing device 10 andthe plate battery 38.The filament battery 37 supplies thgznergy for lighting thefilamen-ts ofthe tu The'circuit terposed between the two tubes inclu the inductance28, the capaciequalizer circuit. The function of the equalizer circuitis to shunt'off from the input circuit of the three-electrode tube 27 toa greater or less degree the lower. frequency currents from the outputof the three electrode tube 26.- The relative amount of low frequency ashigh "frequency currents; which 1s thus shunted can be regulated byvarying either one ofthe three elements in the equalizer :circuit, butpreferably by varying the capacitance 29 or the resistance 30. By

for the operator at will to by-pass practically means .of such anarrangement, it is possible or-biasing battery 25 is shunt-.

grid circuit of the vacuum tube 27 is connected to the potenthrough thecoils 36 of where the echoes are less pronounced, it is possible tointroducevmore of thelow fre Euency tones without harmful interference.onsequently, we find that such a circuit arran ement as just describedis highly desira le in equipping the sound reproducing apparatus invarious types of auditoriums.

Inasmuch as it is highly desirablethat the sound equalizer system of theamplifier be actually embodied in the same, and that ordinaril thisamplifier is 100 ted in the projection oth where it is ve 'flicult orimprac- 'tical for the operator to clearly hearor judge the soundreproduction in the auditorium, we prefer to adjust the equalizercircuit once for all to best adapt the tonal qualities of the sound tothat particular auditorium and not to vary the elements of the equalizerfrom time to time as this or that sound record is I reproduced. However,wedo not wish to be limited in this respect, for it is perfectlyfeasible to install the amplifier or part of it down in the auditoriumwhere the musical conductor or other expert can at will vary the controlelements and thus emphasize .to a greater or less degree the high ortonal qualities of the sound from the loud speaker.

' Instead of the equalizer system as shown in Figure 2, other equivalentarrangements well known in the art may be used. For example, anequalizer system may be inserted in series between the photo-electriccell and the input of the first audion' amplifier, or it may be insertedin series rather than in shunt-between any two audion tubes of theamplifier series, or the equalizer may be inserted directly in theoutput circuit of the last audion tube of the amplifier series. In thislatter case, it is,

of course, easy to locate the same in theauditorium in v shown inFigure 1. We find, however, that lace of the simple rheostat as thereare many advantages in inserting the equalizer system interstage in theamplifier as shown in Figure 2.

We have found in actual practise that with any type of photo electriccell containing gas there is a certain time lag or delay in opera- .tionto full sensitivity of the cell, which lag operates to reduce therelative strength of response to highfrequencylight variations.

compared to that oflow frequency light variations. For example, when thefilm record of a sound of 3,000 to4=,000 vibrations per second passes infront of theslit, thus cutting ofi or reducing the light which falls upon the photo electric cell, 3000 to 4000 times per second the relhtiveapparent changes in resistance of the cell are far less pronounced, thanwhen this same film is run in front of the slit at a much slower speed,so that the lightfluctuations fall upon the cell at the rate of say 500to 1500 vibrations per second. This apparent loss in sensitivity of the'photo-electrlc cell for high frequency light fluctuations may be aphenomena peculiar to the small intensities of light obtained when weemploy such a short or narrow slit as is necessary with the phonofilmdevice, but without going further into the theory or exact explanationof this observed phenomena, the fact remains that there is an apparentlag in gaseous photoelectric cells to high frequencylightlfii'lctuations.

We have discovered the above effect, which was not recognized heretoforeby authorities on the photoelectric cell, and we have discovered apractical method of neutralizing this apparent cell lag or ofcompensating for same, by utilizing the equalizer circuit as describedherein. By this method, the amplifier is made to over-amplify the higherfrequencies or under amplify the lower frequencies to a degreeapproximatelyinversely proportioned to the relative loss in sensitivityof the photo-electric cell with increased frequencies, as abovedescribed.

We, therefore, desire and intend to cover the function of compensatingfor the time lag of the photo-electric cell, as well as adapting thesound reproducing device to the acoustic requirements of any givenauditorium. We are'well aware of the fact that many changes in thedetails of construction and relative ar rangeinent of parts may readilyoccur to those who are skilled in this art, and we do not, therefore,desire to be strictly limited to the particular illustrative embodimentshown and described by us, but rather do we desire to be limited only bythe appended claims.

What we seek to secure by United States Letters Patent is: I

1. In a sound reproducing system, the combination of an amplifier, aloud-speaker connected therewith, an auditorium containing saidloud-speaker, and means controllable at will for adapting the soundoutput from said loud-speaker to the acoustic requirements of saidauditorium, said means including an equalizer vcircuit in said amplifiercircuit, adapted to vary at will the relative intensities of low andhigh frequency currents amplified in said amplifier.

2. In a sound reproducing system, the com- U adapted to vary at will therelative intensities of low and high frequency currents amplified insaid amplifier, for the purpose of adapting the sound output from theloud-speaker to the acoustic requirements of said auditorium.

3. In a sound reproducing system, the combination of a photographicsound record, a photo-electric cell influenced by said sound record, avacuum tube amplifier having a loud speaker connected thereto, and anauditorium containing said loud speaker, said amplifier circuitincluding as an element thereof, an equalizer circuit adapted to amplifycertain of the frequencies of a sound reproduction.-

4. In a sound reproducing system, the combination of a photographicsound record, a photo-electric cell influenced by said sound record, avacuum tube amplifier having. a loud speaker connected thereto and anauditorium containing said loud-speaker, said amplifier circuitincluding as an element thereof, an equalizer circuit adapted to varythe volume of the high frequency sounds without correspondingly varyingthe lower frequency sounds of any sound reproduction.

5. The method of adapting a sound reproduction to the acousticrequirements of a given space which comprises changing light vibrationsincluding a range of frequencies to sound Vibrations and controlling thesound vibrations at will by increasing the volume of the high frequencysounds without correspondingly varying the low frequency sounds.

In testimony whereof we have hereunto set our hands on this 1st day ofJune, A. D. 1926.

LEE DE ronns'r. JOHN Lo IsREYNoLDs.

